Gas turbine engines of the axial flow type conventionally include turbines that are made up of axially alternate annular arrays of radially extending stator aerofoil vanes and rotary aerofoil blades. The demands of modern gas turbine engines dictate that the gases that flow through, and thereby drive, the turbine are at extremely high temperature. As the gases flow through the turbine, their temperature progressively falls as they drive the turbine. However, notwithstanding this, the gas temperatures in the higher pressure regions of the turbine are so high that some form of aerofoil cooling is required.
Conventionally, turbine aerofoils, both blades and vanes, are cooled internally with air that has been tapped from the gas turbine engine's compressor. Using engine compressor air in this manner does, however, carry a penalty in terms of the overall operating efficiency of the engine. Thus generally speaking, the larger the percentage of air taken from the compressor, the greater the adverse effect there is upon overall engine operating efficiency.
Many efforts have been made in the past to make efficient use of compressor-derived air in the cooling of aerofoils. These efforts have centered mainly around the design of aerofoils provided with internal passages for the flow of cooling air. Typically such passages are in a serpentine configuration to provide convection cooling and some of the air from the passages is exhausted through small holes that provide communication between the passages and the aerofoil external surface. As the air is exhausted from the holes, it forms a film that provides additional aerofoil cooling.
Aerofoils cooled in this manner are often complex internally and hence difficult and expensive to manufacture. Moreover, they may not be as effective as is desirable in providing overall aerofoil cooling in view of the air pressure losses that are associated with flowing the cooling air through the many turns in the small diameter passages within the aerofoil.
It has been suggested in FR2,569,225 to provide a hollow aerofoil in which the walls of the aerofoil are provided with radially extending passages. The passages are in communication with the hollow aerofoil interior and also with the external surface of the aerofoil. Cooling air is supplied to the central aerofoil chamber from where it flows into the radially extending passages. From the radially extending passages, it flows on to the aerofoil exterior surface to provide film cooling thereof.
Although such aerofoils are cooled effectively, the pursuit of greater engine efficiency makes yet more effective cooling a highly desirable objective. It is an object of the present invention to provide such an aerofoil.